In general, vacuum glazing is composed of a minimum of two glass panels separated by a void space with a thickness in the range of between 100 μm and 800 μm. Sealing is obtained by a peripheral seal. To achieve super-insulation performances (coefficient of surface transmission U<0.6 w/m2K), the vacuum level between the glass panels must be in the order of 10−3 mbar or less, and generally at least one of the two glass panels must be covered by a low-emissivity layer having an emissivity of ideally less than 0.05.
Different seal technologies exist and each has some disadvantages. A first type of seal (the most widespread) is a seal based on a welding glass, the melting temperature of which is lower than that of the glass of the glazing panels. The use of this type of seal limits the choice of low-emissivity layers to those that are not impaired by the thermal cycle necessary for usage of the welding glass, i.e. to that which is resistant to a temperature that can be up to 350° C. Moreover, since this type of seal based on welding glass has very low deformability, it does not allow absorption of the effects of differential expansions between the glass panel of the glazing on the internal side and the glass panel of the glazing on the external side when these are subjected to substantial differences in temperature (e.g. 40° C.). Quite significant stresses are thus generated on the periphery of the glazing and can cause breakages of the glass panels of the glazing.
A second type of seal comprises a metal seal, e.g. a metal strip with a low thickness (<500 μm) welded around the periphery of the glazing by means of an attachment sub-layer covered at least partially with a layer of a solderable material such as a tin alloy soft solder. A significant advantage of this second type of seal over the first type of seal is that it can be deformed to absorb the differential expansions created between the two glass panels. There exist different types of attachment sub-layers on the glass panel.
Patent application (WO2006/121954) proposes a first exemplary embodiment of a seal of the second type for vacuum glazing. According to this example, the adhesion layer is deposited onto the first glass panel using different processes (physical vapour deposition (PVD), chemical vapour deposition (CVD) and cold spraying). The greatest disadvantage of CVD or PVD deposits is that they are relatively costly and complicated to implement.
For its part, deposition by cold spraying is also costly and complicated to implement and can, moreover, damage the glass substrate, on which it is conducted. In fact, such a deposit can cause cracks on the glass substrate that lead to leakages and a loss of the level of vacuum inside the glazing that does not allow a sufficiently low vacuum level (˜10−3 mbar) to be maintained for the life of the vacuum glazing (generally 10 years minimum).
U.S. patent (U.S. Pat. No. 5,227,206; cf. column 1, lines 60-65) proposes a second exemplary embodiment of a seal of the second type for vacuum glazing. According to this example, the adhesion layer is a copper layer deposited using a low-velocity flame-spraying process. The main disadvantage of this sub-layer is its porosity (U.S. Pat. No. 5,227,206; cf. bridging paragraph of columns 1 and 2). This type of deposit does not allow adequate sealing to be obtained to maintain a sufficiently low level of vacuum (˜10−3 mbar) for the life of the vacuum glazing (generally 10 years minimum).